Acoustical panels are used to form interior surfaces, such as ceiling tiles, wall panels, and other partitions (e.g., partitions between office cubicles), in commercial or residential buildings. The panels are generally planar in shape and include an acoustical layer containing a combination of materials selected to provide suitable acoustic absorbency while retaining sufficient durability. For example, common materials presently used in forming acoustical panels include mineral wool, fiberglass, expanded perlite, clay, calcium sulfate hemihydrate, calcium sulfate dihydrate particles, calcium carbonate, paper fiber, and binder such as starch or latex. Mineral wool is most commonly used because it helps create a porous fibrous structure and thus provides good sound absorption.
Many acoustical panels are prepared in a manner similar to conventional papermaking processes by water-felting dilute aqueous dispersions of mineral wool, perlite, binder, and other ingredients as desired. In such processes, the dispersion flows onto a moving foraminous support wire, such as that of a Fourdrinier or Oliver mat-forming machine for dewatering, as will be appreciated by one of ordinary skill in the art. The dispersion dewaters first by gravity drainage and then by vacuum suction. The wet mat is dried in a heated convection oven, and the dried material is cut to desired dimensions and optionally top-coated with paint to obtain the finished panel. An example of a panel prepared in this manner is the AURATONE® ceiling tile, commercially available from USG Interiors, Inc.
Acoustical panels also can be made by a wet pulp molding or cast process such as described in U.S. Pat. No. 1,769,519. In accordance with this process, a molding composition comprising granulated mineral wool fibers, fillers, colorants, a binder such as cooked starch, and water, is prepared for molding or casting the panel. The composition is placed upon suitable trays that have been covered with paper or a paper-backed metallic foil and then the composition is screeded to a desired thickness with a forming plate. A decorative surface, such as elongated fissures, also may be provided by a screed bar or patterned roll. The trays filled with the mineral wool composition are then placed in an oven to dry. An example of a panel prepared in this manner is the ACOUSTONE® ceiling tile, commercially available from USG Interiors, Inc.
The water felting and tray casting techniques for preparing acoustical panels are not entirely satisfactory because of their complexity and rather significant expense. In addition to raw material costs, these processes expend large amounts of water and energy. Furthermore, many panels prepared according to these methods are subject to unsightly permanent deformation such as sag, especially under conditions of high humidity. In this respect, because many panels are composed of hygroscopic binder such as recycled paper fiber and/or starch, such panels are susceptible to sagging. The possibility of sagging is particularly problematic in the event that the panels are stored and/or employed horizontally. The panels may sag, for example, in areas between the points at which they are fastened to, or supported by, an underlying structure, including, for example, a ceiling grid. The problem of sagging can be more pronounced where the panels must carry loads, including, for example, insulation.
Some acoustical panels are designed to have set gypsum (i.e., calcium sulfate dihydrate) in the acoustical layer. Because set gypsum is not inherently a particularly acoustically absorbent material, many acoustical panels comprising set gypsum include very large mechanically-formed holes that may be, for example, drilled, punched, or otherwise formed to pass through the entire depth of the panel. The holes of acoustical panels of this type typically have a diameter of at least one centimeter, such as found in acoustical panels commercially available from Danoline of Valby, Denmark and from British Gypsum. Many of these panels also utilize an acoustically functional backing sheet. The acoustically functional backing sheet is typically glass fleece or a polymeric material that absorbs or dissipates sound transmitted by the large mechanically-formed holes, but a significant amount of sound is still transmitted through the panel. Although the large mechanically-formed holes provide some acoustical absorbance where there is a plenum behind the panel, many consumers do not find them to be aesthetically pleasing. Gypsum-based acoustical panels having mechanically-formed large holes also are relatively dense products and therefore are cumbersome to transport, to handle, and to install. In addition, the backing sheet adds considerable expense to such products.
More recently, there have been efforts in the art to form acoustical panels from cementitious materials. For example, U.S. Pat. No. 6,443,258 B1 describes an acoustically absorbent porous panel formed individually in a mold from a cured aqueous foamed cementitious material that includes a very low ratio of water to cementitious material (i.e., less than 1:1). Paper fibers are avoided in the panels described in the '258 patent so as to accommodate the low water to cementitious material ratio taught therein. As a substitute for paper fiber, the '258 patent describes the use of polyester, fiberglass, or mineral wool characterized by very long fiber lengths. According to the '258 patent, the length of such fibers is preferably on the order of 0.5 inches so that the fibers can pass through the crystalline cement structure and also pass through the pores created by the foaming process. Thus, acoustical panels prepared according to the '258 patent are expensive to produce and the process for making the panels is inefficient for generating economies of scale.
Accordingly, it will be appreciated from the foregoing that there is a need in the art for acoustical panels that are relatively inexpensive to manufacture and that are produced efficiently in large quantities on a gypsum board line. It will be appreciated also that there is a need in the art for such an acoustical panel that is aesthetically pleasing and does not require the presence of relatively large holes that are mechanically bored into the panel. It will be appreciated further that there is a need in the art for such an acoustical panel that resists permanent deformation, such as sag. The invention provides an acoustical panel and method for the preparation thereof that includes such features. These and other advantages of the present invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.